The present invention relates to a method of measuring the distance from a station of a passive optical star network to the central node of the passive optical star network when the station is put into operation, and to a transmitting and receiving station suitable for carrying out the method.
The invention starts from a communication system having a plurality of transmitting and receiving stations which are interconnected by a passive optical star network. The transmitters of these stations transmit information to the central node of this star network, from where the transmitted information is distributed to the receivers of all stations. To avoid mutual interference, the stations are assigned individual time slots. The manner of this assignment does not form part of this invention, which, in turn, is independent of the manner of the assignment. What is important is that the assignment takes place in advance, even though only shortly before.
The network is to be operated at such a high transmission speed that the delay cannot be left unconsidered. The delay can be taken into account by corresponding guard time intervals between the transmission of the individual stations. It is desirable to keep the guard time intervals shorter than the delays. This can be achieved by allowing for the various delays. In an optical star network, the transmissions of the individual stations follow separate paths up to the central node, are united there, and then go as a complete frame to all stations involved. The frame is thus formed at the central node. To permit correct frame formation, each transmitting station must take into account the propagation time to the central node and transmit correspondingly earlier. This requires that this propagation time be known.
Measuring the lengths of the cables laid to the central node would in practice be unsatisfactory, and the measurement result could not be automatically adopted by the station connected or to be connected to the network.
Prior to system start-up, an echo measurement could be performed at the individual stations, and its result could be used direct. However, when a new station is added or a station is put into service again after an interruption, e.g., for a repair, that would not be possible without interfering with the operation of the other stations.
Disconnecting the measuring station from the central node is undesirable for practical reasons and, on the other hand, requires that only one station be connected to each ray of the star. However, neither the connection of two or more stations to one ray nor any branching of individual rays is to be excluded.